This invention relates to systems and methods for wirelessly projecting power and more particularly to systems and methods for wirelessly projecting power to microelectronic devices.
Wireless powering of microelectronic devices is used, for example, for wireless Radio Frequency (RF) powering of Radio Frequency Identification (RFID) tags. RFID tags are used in the Automatic Data Collection (ADC) industry. In particular, printed bar codes are now widely used in the ADC industry. Unfortunately, bar codes may require line of sight reading, may hold limited amounts of information, may need to be read one at a time, may be subject to defacing and/or counterfeiting and may only provide fixed information. In contrast, RFID tags need not require line of sight reading, can hold large quantities of information, can have high transfer data rates, can be read in groups, can be more reliable and more difficult to destroy and/or counterfeit and can update stored information.
RFID tags generally may be classified into battery powered (active) RFID tags and RF powered (passive) tags. Compared to passive tags, active tags may be more expensive, may have a defined shelf life, may deplete with operation, may have potential disposability problems, may be physically larger and may be environmentally constrained due to the presence of a battery thereon. In sharp contrast, passive tags can be less expensive, can have an unlimited shelf life without depletion, can be relatively safe to dispose, can be relatively compact and can withstand harsher operating environments.
Notwithstanding these potential advantages, a major factor that may limit the availability of passive RFID tags is the ability to wirelessly project sufficient power to power the RFID tag.
In particular, RF communication among electronic devices currently is used across the RF spectrum. For example, cellular radiotelephones are widely used. In the United States, the Federal Communications Commission (FCC) regulates usage of electromagnetic radiation.
Unfortunately, the amount of power that is used to operate electronics may be orders of magnitude more than is used to exchange information. Accordingly, notwithstanding the advent of low power microelectronic devices, the ability to transmit enough power to be extracted by a remote microelectronic device may be difficult. In wirelessly projecting power to wirelessly power microelectronic devices, the biggest constraint may be the government regulations concerning permissible RF field strength.
Electromagnetic field emanation from an antenna classically is categorized as xe2x80x9cnear fieldxe2x80x9d and xe2x80x9cfar field.xe2x80x9d Generally, electronic components that carry RF currents or voltages produce both types of fields. However, the relative amount of each field may vary greatly.
From an RF energy standpoint, near field generally refers to RF energy that is stored in the immediate vicinity of the component and that is recovered at a later time in the alternating RF current cycle. An ideal inductor is a perfect near field only device. Far field generally refers to the energy that radiates or propagates from a component as an electromagnetic wave. Thus, a real world inductor may produce some far field radiation. Conversely, an ideal dipole antenna produces no near field components but produces significant far field radiation. Real world dipole antennas may produce some near field components but generate large amounts of far field radiation.
Thus, the far field is the component of energy that permanently leaves an antenna or any other component, radiating or propagating into the environment as an electromagnetic wave. Conversely, in each cycle, a near field is created and the energy associated with the near field is stored in the space around the antenna. As the near field collapses, the energy is transferred back onto the antenna and drive circuitry.
It will be understood that the terms xe2x80x9cnear fieldxe2x80x9d and xe2x80x9cfar fieldxe2x80x9d classically also may be defined relative to the wavelength of the energy under consideration. As used herein, far field denotes energy at distances greater than about one wavelength, for example, greater than about 22 meters at 13.56 MHz and greater than about 31.6 cm at 950 MHz. Conversely, near field refers to energy that is less than about one wavelength in distance. For practical purposes, near field generally may be considered to be a fraction of a wavelength, while far field may generally be considered to be multiple wavelengths so that there may be an order-of-magnitude difference therebetween.
Near field and far field also may be distinguished by the drop-off of power from the antenna. Power in the far field generally drops off from a source antenna without gain as a function of 1/(distance)2. In contrast, power in the near field generally may exhibit a more complex relationship. At distances that are far less than one wavelength, the individual current carrying elements of the antenna may produce a near field that decreases, remains constant or may even increase with distance. Moreover, at distances that approach one wavelength, power generally drops off much quicker with distance compared to the far field, with some components dropping off as fast as 1/(distance)8, others closer to 1/(distance)4.
Antennas generally are designed to communicate over great distances. Accordingly, antennas generally are designed to optimize the far field for a particular application. Accordingly, FCC regulations also generally are written for far field radiation. For example, radiation typically is measured based on FCC standards at a distance greater than one wavelength because it is assumed that near field energy is greatly reduced at that distance. However, there also are FCC guidelines that relate to maximum exposures to electromagnetic radiation that can impact near field intensity limits.
For purposes of wirelessly projecting power to wirelessly power microelectronic devices, it would be desirable to increase the near field component of energy without increasing the far field component of energy sufficiently to violate FCC regulations. Preferably, the near field component also is not increased to the point where maximum exposure as stated by the FCC guidelines occurs too quickly. By increasing the near field component of energy, the microelectronic devices may be powered by the field that is stored in the space around the radiator. By not increasing the far field, the energy that propagates outward and that is not reclaimed may be reduced, and violation of government regulations that govern far field energy may be prevented. Unfortunately, when the near field is increased in order to extend the range at which power may be projected to wirelessly power microelectronic devices, the far field also may increase, thereby increasing the likelihood of regulatory violations.
It is therefore an object of the present invention to provide systems and methods for wirelessly projecting power to wirelessly power microelectronic devices.
It is another object of the present invention to provide systems and methods that can project power to wirelessly power microelectronic devices over longer distances, and can reduce the likelihood of violating regulatory constraints.
These and other objects can be provided according to the present invention by an array of in-phase current loops that are disposed adjacent to one another to define a surface and to define a virtual current loop at a periphery of the surface that produces a same direction virtual current while current in adjacent portions of adjacent current loops flows in opposite directions, to thereby wirelessly project power from the surface. It has been found according to the invention that the array of in-phase current loops that are disposed adjacent to one another to define a surface and to define a virtual current loop at a periphery of the surface that produces a same direction virtual current while current in adjacent portions of adjacent current loops flows in opposite directions, can provide acceptable power to RFID tags, while reducing the risk of violating regulatory constraints.
It will be understood that, as used herein, the terms xe2x80x9cin-phasexe2x80x9d, xe2x80x9csame directionxe2x80x9d, xe2x80x9copposite directionxe2x80x9d and xe2x80x9cout-of-phasexe2x80x9d refer to relationships at a given point in time. In particular, since alternating currents are used, that vary over time, the terms xe2x80x9cin-phasexe2x80x9d, xe2x80x9csame directionxe2x80x9d, xe2x80x9copposite directionxe2x80x9d and xe2x80x9cout-of-phasexe2x80x9d refer to instantaneous current relationships. Moreover, it also will be understood that the terms xe2x80x9cin-phasexe2x80x9d, xe2x80x9csame directionxe2x80x9d, xe2x80x9copposite directionxe2x80x9d and xe2x80x9cout-of-phasexe2x80x9d refer to current loops that are substantially in phase and virtual currents that are substantially in the same direction or substantially in opposite directions or out-of-phase. For example, current loops that are within xc2x120xc2x0 of one another, more preferably xc2x110xc2x0 of one another and most preferably of identical phase may be considered xe2x80x9cin-phase.xe2x80x9d Virtual currents that are within xc2x120xc2x0 of the same direction, more preferably within xc2x110xc2x0 of the same direction and most preferably identically in the same direction may be considered to be in the xe2x80x9csame direction.xe2x80x9d Finally, current loops or currents that are within 180xc2x0xc2x120xc2x0 of one another, more preferably 180xc2x0xc2x110xc2x0 of one another and most preferably 180xc2x0 out-of-phase with one another may be regarded as being in the xe2x80x9copposite directionxe2x80x9d or xe2x80x9cout-of-phase.xe2x80x9d
Without being bound by any theory of operation, the present invention may be explained by dividing the classical near field as described above, into a xe2x80x9cclose-in near fieldxe2x80x9d and a xe2x80x9cmid fieldxe2x80x9d. The close-in near field refers to RF energy that is stored in the immediate vicinity an antenna, up to a distance of about the dimension of the antenna, such as the length of a dipole or the diameter of a loop. The mid field refers to RF energy that extends beyond the distance of about the dimension of the antenna to a distance of about one wavelength. Thus, for example, for a ten-inch diameter loop antenna that is radiating at 13.56 MHz, the close-in near field may extend from the plane of the loop to a distance of about ten inches, the mid field may extend from a distance of between about ten inches to about 22 meters and the far field may extend to distances that are greater than about 22 meters. It will be understood, however that since both the close-in near field and the mid field are components of the near field, they both comprise RF energy that is stored and recovered in the alternating RF current cycle.
It has been found, according to the invention, that an array of in-phase current loops that are disposed adjacent to one another to define a surface and to define a virtual current loop at a periphery of the surface that produces a same direction virtual current while current in adjacent portions of adjacent current loops flows in opposite directions, can reduce far field radiation so that the likelihood of violations of government regulations can be reduced. Moreover, by reducing far field radiation, the current in the array of in-phase current loops that are disposed adjacent to one another to define a surface and to define a virtual current loop at a periphery of the surface that produces a same direction virtual current with current in adjacent portions of adjacent current loops flowing in opposite directions, can be increased to thereby allow an increase in mid field components. Sufficient mid field components to power RFID tags thereby may be provided, without violating government regulations concerning far field radiation.
In one preferred embodiment of the present invention, the array of in-phase current loops comprises an array of at least three wedge-shaped current loops each having an outer portion and a pair of sides. The at least three wedge-shaped current loops are disposed adjacent to one another to define a surface such that the virtual current loop defined by the outer portions flow in same directions and current in adjacent sides of adjacent current loops flow in opposite directions. The wedge-shaped current loops may be identical or mirror imaged. Two wedge-shaped current loops also may be provided, wherein each wedge is semicircle-shaped.
In another embodiment, the array of in-phase current loops comprises an array of at least two polygonal current loops, such as hexagonal current loops, each having a plurality of sides. The at least two polygonal current loops are disposed adjacent to one another to define a surface and a virtual current loop at the periphery of the surface that produces a same direction virtual current, with current in adjacent portions of adjacent current loops flowing in opposite directions. Stated differently currents in the sides of the at least two polygonal current loops that comprise the outer boundary are in-phase and currents in adjacent sides of adjacent current loops are out-of-phase. In yet another embodiment, the current loops may be circular or elliptical in shape.
In all of the above-described embodiments, the surface preferably is a planar surface. However, non-planar surfaces such as spheroidal surfaces also may be used. The surface may be a physical surface in which the array of in-phase current loops are mounted or may be a virtual surface defined by the array of in-phase current loops. Each of the current loops may be a spiral current loop, a concentric current loop and/or a stacked current loop. The length of each current loop preferably is less than a quarter wavelength.
In a preferred embodiment, a driver drives the array of current loops at 13.56 MHz to thereby wirelessly project power. The frequency of 13.56 MHz preferably is used because the FCC allows relatively large amounts of field strength at this frequency. In particular, in the range of 13.56 MHzxc2x17 KHz, FCC regulations allow 10,000 xcexcV/m, whereas immediately outside that range only 30 xcexcV/m may be allowed. However, other frequencies also may be used in the United States and in other countries.
In order to allow further reduction of the far field electromagnetic waves and further increases in current to provide additional mid field electromagnetic field strength, a plurality of arrays of in-phase current loops may be provided. The multiple arrays of in-phase current loops are disposed adjacent to one another to define a surface. Each array of in-phase current loops may be configured as was described above.
In a preferred embodiment that uses multiple arrays of in-phase current loops, the virtual current loops of adjacent arrays of in-phase current loops produce different phase virtual currents from one another. Specifically, four arrays of in-phase current loops may be provided that are arranged in two rows and two columns, such that the virtual current loops in the arrays in each row and each column are of opposite phase. In another embodiment, the virtual currents in the arrays in each row and each column are approximately 90xc2x0 out-of-phase from one another. The two rows and columns may be orthogonal or non-orthogonal. Preferably, the two rows and two columns are obliquely arranged relative to the horizontal so that a tag passing across the plurality of arrays in the horizontal direction will encounter varying fields to thereby increase the likelihood of receiving sufficient power.
In another embodiment, six arrays of in-phase current loops may be provided that are arranged in four rows and two columns. In the first row, the phases of the virtual currents of the two arrays differ by approximately 60xc2x0. In the second row, the virtual currents of the two arrays flow in same directions, and in the third row, the phases of the virtual currents of the two arrays differ by approximately 60xc2x0. Viewed along the first column, the phases of the virtual currents are approximately 0xc2x0, 120xc2x0 and 60xc2x0 and along the second column the phases of the virtual currents are approximately 60xc2x0, 120xc2x0 and 0xc2x0.
In another embodiment, a plurality of arrays of in-phase current loops are arranged in a circle, such that the virtual currents in adjacent arrays in the circle are of opposite phase. Alternatively, the phases may differ by approximately 360xc2x0/n, where n is the number of arrays of in-phase current loops that are arranged in a circle. The plurality of in-phase current loops also may be arranged in an elliptical shape or a polygonal shape. They may be overlapping or spaced apart.
Accordingly, reduced far field radiation may be produced by systems and methods according to the present invention. By producing reduced far field radiation, the current in the current loops may be increased to thereby increase the mid field strength without violating government regulations for far field radiation. In order to reduce the close-in near field without significantly changing the mid field or far field, the outer portions of the wedge-shaped current loops also can be implemented as multiple loops that are spatially separated, while the sides of the wedges can remain the same. Thus, the close-in near field may be reduced so that exposure time under FCC guidelines can be increased.
Having provided systems and methods for wirelessly projecting power to wirelessly power microelectronic devices, other problems in RFID tags also may be solved according to the present invention. These solutions can provide improved systems and methods for identifying a plurality of RFID tags that are simultaneously interrogated on a single communication channel. As is well known to those having skill in the art, RFID tags may conflict with one another when responding to a single RFID reader since the tags transmit on the same frequency and within the same time slot. Thus, multiple responses may be generated in response to a single request to read multiple tags.
It is known to use a binary tree in order to bitwise interrogate the activated tags. Unfortunately, the use of a binary tree may be too slow and may be dependent on maintaining a high level of synchronization for extended periods. Moreover, tags entering or leaving the volume during traversal of the tree may not be addressed.
According to one aspect of the present invention, each identification tag comprises at least four identification bits. At least two identification bits in each of the identification tags are simultaneously interrogated to obtain a predetermined response from each of the identification tags having a predetermined bit value for each of the at least two identification bits. Thus, an N-ary tree is used rather than a binary tree where N is three or more. Higher identification speeds thereby may be obtained.
Speed also may be further increased, by not traversing the tree down to all of its nodes. Rather, once the tree is traversed at least part way so that the number of possible tags to be identified is reduced, each of the tags having a predetermined bit value for at least two identification bits may be serially interrogated to obtain at least two additional identification bits from each of the identification tags having a predetermined bit value for the at least two identification bits. The combination of multiple bit simultaneous interrogation and multiple bit serial interrogation can produce efficient methods and systems for identifying a plurality of RFID tags that are simultaneously interrogated on a single communication channel.
Finally, the present invention can provide efficient methods for traversing the tree of identification bits. In particular, a tree of identification bits is defined, the tree comprising a plurality of nodes at a plurality of levels that define differing values for subsets of the plurality of identification bits. A first subset of a plurality of identification bits in each of the identification tags that correspond to a first node at a first level of the tree are simultaneously interrogated to obtain a predetermined response from each of the identification tags that correspond to the first node at the first level of the tree. If at least one tag in the first subset responds, a xe2x80x9cpushxe2x80x9d command may be used to descend the tree one level, to simultaneously interrogate a second subset of a plurality of identification bits in each of the identification tags that correspond to a second node at a second level of the tree, to thereby obtain a predetermined response from each of the identification tags that correspond to the second node at the second level of the tree. For N-ary trees, N distinct push commands may be used to identify a unique path in the N-ary tree.
A xe2x80x9cpopxe2x80x9d command may then be used to ascend one level in the tree. Another xe2x80x9cpushxe2x80x9d command may then be used to descend an alternate branch of the tree by simultaneously interrogating a third subset of the plurality of identification bits in each of the identification tags that correspond to a third node at the second level of the tree to obtain a predetermined response from each of the identification tags that correspond to the third node at the second level of the tree. Thus, by using xe2x80x9cpushxe2x80x9d and xe2x80x9cpopxe2x80x9d commands, the tree may be navigated efficiently and need not be repeatedly navigated from top to bottom. Accordingly, the data in the tags that are powered can be ascertained quickly and accurately. It will be understood that, as used herein, the terms xe2x80x9cfirst,xe2x80x9d xe2x80x9csecondxe2x80x9d and xe2x80x9cthirdxe2x80x9d refer to any three relative levels of a tree, and do not refer to absolute levels.